It is well known by those skilled in the art that internal combustion engines burning a lean mixture of fuel and air consume significantly less fuel than when operating at a stoichiometric mixture of air and fuel. Presently, there are very few lean burn gasoline engines in production due to difficulties in meeting emission requirements. The difficulty is that conventional precious metal catalyst oxidizes CO and hydrocarbons and reduces NOx at high efficiency when the air-fuel mixture is very close to stoichiometric; but, NOx conversion efficiency drops off substantially when the exhaust gases are lean.
It is known in the art to use a lean NOx trap (LNT) aftertreatment system for processing the products of lean combustion. During lean combustion, NOx is trapped in the LNT. When the LNT is full, the engine is operated rich for a short period of time. The rich exhaust gases cause the absorbed NOx to desorb from catalyst surfaces. Furthermore, the rich exhaust gases contain CO and unburned hydrocarbons that reduce NOx to N2. Although commonly called a lean NOx trap, the LNT actually stores only NO2 to a high degree. Because NOx coming from the engine is predominantly comprised of NO and very little NO2, an oxidation catalyst is provided upstream to cause NO to oxidize to NO2.
The inventors of the present invention have recognized a difficulty in relying on an oxidation catalyst to perform the oxidation of NO to NO2. Specifically, the oxidation catalyst is only partially active below a temperature of about 200° C. Thus, during warm up or at very low power conditions, the reaction from NO to NO2 is marginal. Consequently, NO proceeds through the LNT and out the vehicle tailpipe unprocessed.
A known problem with lean NOx traps is their susceptibility to SOx contamination. Most hydrocarbon fuels contain some sulfur. The sulfur oxidizes mostly to SO2 during the combustion process in the combustion chamber. If an oxidation catalyst is placed upstream of the LNT, the SO2 is further oxidized to SO3. SO2 can pass through the exhaust system with no harmful effect. However, SO3, in the presence of water vapor in the exhaust, forms particulates containing sulfuric acid. These become absorbed in the LNT and reduce its conversion efficiency. To overcome sulfur degradation of LNT performance, it is known to periodically regenerate the trap, commonly called deSOx. The SOx can be desorbed and made to exit the LNT when its temperature is raised to a high temperature, in the range of 700-800° C., for a period of time, typically greater than a minute. The inventors of the present invention have recognized several problems with sulfur contamination: first, the LNT operates at less than its optimal efficiency for much of the time due to the sulfur contamination and secondly, the deSOx operation is cumbersome, penalizes fuel economy, and the deSOx temperature is near the temperature at which permanent damage to the LNT occurs making control of deSOx regeneration a challenge. Furthermore, deSOx regeneration is not completely reversible. The propensity of an oxidation catalyst to oxidize SO2 to SO3 is harmful to the LNT. Some LNTs contain precious metals, such as platinum, in their formulation. In such LNTs, the oxidation of SO2 to SO3 happens regardless of whether there is an oxidation catalyst upstream or not.
The inventors of the present invention have further recognized that it is desirable to provide any aftertreatment system with an onboard diagnostic procedure to detect system deficiencies.